Nowadays, a power generation system, in which a photovoltaic cell, a fuel cell, and the like having less influence on an environment are used, is actively developed from the viewpoint of global environment protection.
In some of the power generation systems, a DC power from DC power sources such as the photovoltaic cell, which generate an electric power, is converted into an AC power having a commercial frequency that is linked to a system by a power conditioner. The post-conversion AC power is supplied to a domestic load connected to a commercial electric power system. On the other hand, an excess power flows back to a system side when the electric power generated by the DC power source is higher than a consumed power of the domestic load.
The power conditioner used in the power generation system generally includes an inverter that converts the DC power generated by the photovoltaic cell and the like into the AC power and a protection device that is used in system linkage. The power conditioner is roughly divided into an insulation type in which a DC unit and an AC unit are electrically insulated by an insulation transformer and a non-insulation type in which the insulation transformer is not used. The non-insulation type is frequently used because the non-insulation type is excellent in power conversion efficiency compared with the insulation type (for example, see Patent Document 1).